Growth of high quality thin films of compound semiconductor on substrates with high lattice and thermal expansion coefficient mismatch with the epitaxial thin film layer, such as III-V nitrides on Si, SiC and sapphire substrates, has shown to be very challenging.
III-V nitride is a field of intense research due to its wide applications for optoelectronic and electronic devices, such as blue and UV light emitting diodes (LEDs) and Laser diodes (LDs), UV detectors and electronic devices such as bipolar transistors. Compared with the currently used and available devices on the market, nitride devices have marked advantages in several aspects. For example, nitride blue laser can provide at least a 400% increase in data storage density on a CD RAM due to its shorter wavelength than those of red and near infrared lasers. On the other hand, the large bandgap of III-V nitride materials make them good candidates for high power and high temperature transistor applications.
Currently, one of the main remaining problems of III-V nitride materials is the unacceptable high dislocation density due to large lattice and thermal expansion coefficient mismatch between them and substrates (e.g. 16.09% and 17% between GaN and sapphire, and GaN and Si(111), respectively, the most widely used substrates for III-V nitrides). High dislocation density dramatically degrades the performance and reliability of nitride devices and shortens device lifetime.
In prior methods, a low temperature AlxInyGa1-x-yN (0<x, y<1) buffer layer is used, as described by H. Amano, et al, Appl. Phys. Lett., Vol. 48, 35 (1986) and S. Nakamura, Jpn. J. Appl. Phys Vol. 30, L1705 (1991), which has dramatically improved the III-V nitride epilayer quality in terms of morphology, electrical and optical properties. Other methods such as lateral epitaxial overgrowth (LEO) and Pendeoepitaxy (PE) have also shown success in reducing the dislocation density. However, both the LEO and PE methods require several additional processing steps before a low dislocation epitaxial layer of the material can be obtained. Despite the use of a buffer layer as proposed by Nakamura et al. and Amano et al., the dislocation density in as-grown nitride epilayers is still very high (108 to 109 cm−2) as diagrammatically illustrated in epilayer 10 on substrate 12 in FIG. 1, which limits nitride device performance and applications.
To solve this problem, it has been proposed to use a compliant universal (CU) substrate by Lo in U.S. Pat. No. 5,294,808 and Hwang et al in U.S. Pat. No. 6,406,795. If an epilayer is grown on a very thin substrate, the misfit dislocations (due to both lattice and thermal expansion coefficient mismatch) will propagate into and are contained in the thin CU substrate rather than the epilayer since it is energetically more favorable. An example of a layer 14 of AlxInyGa1-x-yN grown on a Si SOI (Semiconductor On Insulator) substrate 16, is schematically shown in FIG. 2, in which the thin Si overlayer 18, above a buried oxide layer 20, serves as a thin free-standing substrate. If the thickness of the Si overlayer is lower than its critical thickness, an ideal CU substrate is achieved and the stress between the overgrown epitaxial AlxInyGa1-x-yN layer and the thin Si overlayer will not exceed the critical value that leads to the generation of dislocations at their interface.
The advantage of using a CU substrate is that it is universal and different epilayers can be grown on the same CU substrate. However, the disadvantages of this approach for growth of Ill-V Nitride materials include:
(1) For AlxInyGa1-x-yN/Si heterostructure the critical thickness of Si is around one monolayer, therefore, no ideal CU substrate can be fabricated in this case. With the typical Si overlayer thickness of about 50 to 200 nm, some dislocations will be generated at the interface some of which will penetrate into the overgrown epilayer 14, as schematically illustrated in FIG. 2.
(2) The thin Si overlayer on the top of SiO2 may not give enough guiding force to epitaxial growth of polycrystalline AlxInyGa1-x-yN seeding layer (buffer layer) which leads to a low crystal quality in the buffer layer. A thin Si overlayer does not support epitaxial relationship (lattice structure and orientation) needed in high quality AlxInyGa1-x-yN buffer layer. Quality of the subsequent III-V Nitride layers depend highly on the quality of the seeding (buffer) layer.
Other techniques have been suggested to reduce dislocation density in a top monocrystalline layer as proposed by Bisaro (U.S. Pat. No. 5,141,894) by Mantl (U.S. Pat. No. 6,464,780 B1) and Ramdani (U.S. Pat. No. 6,392,257 B1).
In these earlier patents, monocrystalline buffer layers have been used. However, growth of a monocrystalline AlInGaN thin buffer layer on Si substrate is highly improbable due to the high lattice and thermal mismatch between the two layers and therefore is not suitable in this application.
The Bisaro patent further describes an embodiment in which a monomolecular preliminary layer is deposited on a substrate, e.g. silicon. The substrate is then implanted through the preliminary layer to create a highly disturbed or even amorphous zone on the surface of the silicon. It is also mentioned that the preliminary layer can also be a thicker amorphous layer. The preliminary layer is used to stabilize the silicon surface and to protect the silicon in the implantation stage only. It does not play the role of a seeding layer for the following monocrystalline layer growth after ion implantation.
Further, ion implantation will usually cause significant damage to a monocrystalline layer. In the Bisaro patent, it is not mentioned at all how to keep the buffer layer monocrystalline during ion implantation process. In the Mantl patent, light element (hydrogen) ion implantation with low dose is suggested in order to minimize the damage to the monocrystalline buffer layer, which is feasible, but it is not applicable for high dose ion implantation as needed for this application. Also in the Mantl approach, the monocrystalline epitaxial layer is not disconnected “mechanically” from its substrate and, therefore there is always stress and dislocations present in the layers if the layer thickness exceeds critical dimension.
In Ramdani, an amorphous intermediate oxide layer is generated by thermal diffusion of oxygen through the thin monocrystalline oxide/nitride accommodating buffer layer, eventually reacting with the monocrystalline substrate at the interface to create the oxide layer. Thermal diffusion of oxygen at temperatures of 400° C.-600° C. (as suggested by Ramdani in column 8, line 62) through a monocrystalline layer of GaN with a practical thickness of 20 nm is a very lengthy procedure.
Thus, a need persists for a practical and cost effective method for forming a highly dislocation free compound semiconductor such as AlxInyGa1-x-yN, on a lattice mismatched substrate.